A hygrometer capable of operating under a wide range of temperature and pressure conditions can be constructed by using a sheet of solid hygroscopic-electrolyte material and placing electrodes in contact with opposite faces of this sheet. One electrode has numerous openings through which moisture can pass to be absorbed by the electrolytic sheet. When a voltage is applied between the electrodes, the current electrolyzes the moisture to gaseous hydrogen and oxygen. The amount of current is a direct measure of the moisture which has been absorbed. Thus, moisture can be absorbed over a period of time and a voltage then suddently applied, or the device can be operated under steady state conditions. A hygrometer of this type is described in U.S. Pat. No. 4,083,765 by Lawson.
Several problems arise in constructing and operating this type of hygrometer. One problem is providing a relatively simple and rugged arrangement for applying electrodes to the electrolytic sheet and maintaining good contact of the electrodes therewith during slight expansion and contraction of the electrolytic sheet as it gains and looses moisture. Another problem is maintaining the electrode that has openings therein, in contact with the electrolytic sheet without considerable gaps in such contact, to assure that considerable current passes through all areas of the sheet. Still another problem is avoiding fringing effects near the edges of the sheets, where moisture may be absorbed but through which it is difficult to pass current. A solid electrolytic hygrometer which avoided these problems would therefore be of considerable value.